This invention concerns radiation therapy, especially brachytherapy, for treating tissues which may have diffuse proliferative disease. In brachytherapy, the radiation source is generally placed within a surgically created or naturally occurring cavity in the body. In particular, this invention relates to an applicator for delivering radiation therapy to a vaginal cavity and/or to adjacent tissue, often following surgical treatment of cancer. Radiation therapy of this sort is generally administered over a period of time in partial doses, or fractions, the sum of which comprises a total prescribed dose. This fractional application takes advantage of cell recovery differences between normal and cancerous tissue whereby normal tissue tends to recover between fractions, while cancerous tissue tends not to recover.
In brachytherapy, a prescribed dose is selected by the therapist to be administered to a volume of tissue (the target tissue) lying outside the treatment cavity into which the radiation source will be placed. Generally the prescribed dose will include a minimum dose to be delivered at a preferred depth outside the treatment cavity (the prescription depth). Since, in accordance with the laws of physics, radiation intensity falls off with increasing distance from the radiation source, it is desirable to create and maintain a space between the source of radiation and the first tissue surface to be treated (generally the cavity wall since the source is placed within the cavity) in order to moderate the absorbed dose at the cavity surface. Although not always the case, generally the absorbed dose at the prescription depth outside the cavity is to be uniform. In this isotropic case, it is therefore important that the incident radiation on the interior surface of the cavity be the same at all points being treated. To accomplish this objective, it may be necessary to sequentially position a single radiation source through a series of positions (or utilize multiple sources strategically placed) which, in the aggregate, produce a uniform absorbed dose incident on the cavity surface being treated. When this is achieved, the absorbed dose reaching into tissue will be the same at all points being treated, and the minimum prescribed dose can be delivered at the prescription depth as nearly as the treatment plan will allow. Furthermore, by selecting the radiation source intensity (radioisotope emissions or x-ray tube output) and controlling treatment time and the distance from the source(s) to the cavity interior surface, the incident radiation can be sufficiently moderated to avoid substantial damage to normal tissue.
Rigid applicator cylinders designed to receive radioisotopes have traditionally been used to treat vaginal cancer or malignancies in adjacent tissues. A principal function of an applicator is to establish and maintain distance relationships between the radiation source and the tissues being treated such that the prescribed dose is delivered to a desired prescribed depth of tissue, and yet normal tissues nearest the radiation source are not subjected to absorbed doses sufficient to risk significant necrosis. Applicators of this general type are available, for example, from Varian Medical Systems, Inc., Charlottesville, Va. Such prior art applicator cylinders are sized to the vaginal cavity or adjacent anatomy, but because the tissues should be positioned closely against the exterior surface of the applicator, large applicators must be chosen that are often painful on insertion, and once inserted may fail to provide a good fit. Additionally, prior art cylinders are generally straight, with a central lumen into which radioactive seeds are delivered and later removed after completion of prescribed therapy. As a result, anisotropic treatment plans are difficult to achieve with such symmetrical applicators. Thus conventional applicators are less than ideal in many cases.